Have you ever noticed those giant wing-shaped things on the backs of race cars? They’re called “spoilers” (or sometimes just “wings”), and they’re not just there because they look so darn cool. Spoilers on racecars are an important safety feature and have to be carefully made to make sure that the racecar doesn’t fly up into the air! To understand why, we can do a couple of simple experiments with moving air.
- A blow dryer
- A ping-pong ball
- A piece of printer paper or notebook paper
- Let’s start with the blow dryer. Plug it in and turn it on. Either “low” or “high” setting should work. Hold your hand in front of it to feel the fast-moving air coming out the end.
- Hold the blow dryer so it is pointing straight up. Place the ping-pong ball in the stream of air. It will start to hover as the air pushes up on it!
- Now for the cool part: tilt the blow dryer from side to side, so the stream of air isn’t straight up anymore. If you’re careful, and you don’t go too far, the ping-pong ball will still float, even though, the air isn’t pushing up on it anymore! See how far you can tilt the blow dryer before the ball falls out. Try turning it off while the ball is hovering. What happens?
- Now let’s move on to the paper. Grab it on the long sides closer to one end than the other. Then hold the end close to your hands up to your mouth like you’re going to eat a sandwich. Most of the paper should be drooping downward in front of you.
- Blow gently on the top side of the paper. With some practice, you’ll be able to make that droopy part float upward even though you’re blowing on top of it!
So what’s going on here? We have a ping-pong ball that seems to levitate even when the air isn’t pushing straight up into it, and we have a paper that moves up when you blow over the top instead of the bottom. Believe it or not, these experiments are very similar, and they both have a lot to do with why racecars need spoilers. It’s all about air pressure, or how much air presses on the things around it.
In the 1700s a scientist named Daniel Bernoulli discovered that when air is moving fast, it presses on things less. This means that slow-moving air, or air that doesn’t move at all, presses on things more than air that is moving fast. Fast moving air creates low air pressure; slow moving air creates higher air pressure. In honor of Daniel Bernoulli, this is called “Bernoulli’s Principle.”
The stream of fast-moving air coming out of your blow dryer, then, is low-pressure air. The air around it, which isn’t moving at all, has more pressure, and that unmoving air presses in on the fast-moving air and the ping-pong ball inside it. It presses so hard that it can hold the ping pong ball in place even if the fast-moving air isn’t pushing it directly up.
When you blow on the paper, you’re doing something similar. You’re creating fast-moving air above the paper, which has lower air pressure than the unmoving air below it. The air on the bottom presses more on the paper, so it pushes it up.
Race cars use spoilers to slow down the air that’s moving over the top of them. Depending on the shape of the car, the shape of the track, and the way the air is moving around it, it’s possible that the air moving over the car might move faster than the air moving under the car. If the air above the car moves too fast, then the slower air underneath will press up enough to make the car lift off the ground, which is very dangerous to the driver. A good spoiler breaks Bernoulli’s Principle by slowing that air and keeping all four wheels firmly on the ground.